A few comments:

(1) From multiple personal experiences, watching checkpoints, and what I've repeatedly read here, the "pat down" that occurs after an AIT anomaly (the "yellow boxes") is normally quite brief (2-4 seconds) and isolated to the area of the box. So a request for a full pat-down in such a situation is unusual. We do know multiple anomalies can either indicate a problem with the machine or does require a full pat-down, but have been told that those details are SSI. I've never seen it happen, though, so it's certainly uncommon.

(2) Tearing up or surrendering a boarding pass proves nothing about intent to fly. They are not "accountable documents" and you can trivially get as many copies of it as you want.

(3) Akuai (sp?), which is the 9th Circuit decision mentioned, is almost certainly not relevant in this situation for two reasons. One, as stated, is that it talks about a very different type of search. Second, it only addresses the issue of the exclusionary rule, where any evidence found in a subsequent search for while verbal consent was not given is admissible. It does not at all address the issue of whether somebody can be physically compelled to be searched. In Akuai, the person, while verbally not consenting to the search, physically permitted it (he had drugs in aluminum foil in his socks).

(4) TSA management knows about (3) and does not want to see a precedent come down against them, so does not actually try to view refusal to complete screening as something they'll fine.

(5) It is not clear that "making a u turn in front of a checkpoint" is probable cause or reasonable suspicion of anything. The case that establishes the legality of DUI checkpoints (Michigan Department of State v. Sitz, 496 US 444) seems to imply the opposite. Certainly, no probable cause or reasonable suspicion can legally follow from somebody exercising their 5th or 5th Amendment rights.